A couple of months ago, I posted on a 2004 study by the University of Westminster in the UK that claimed that lavender is toxic to human cells. Because I haven't ever been able to find corroborating evidence, I've decided not to get too paranoid and or practice lavender avoidance. Then I noticed that the same team was making claims in 2006 that clove is toxic. I immediately remembered having a severe allergic reaction to a toothpaste containing cinnamon and clove (I woke up one morning with Pamela Anderson's lips and a stinging pain in my gums; the skin eventually turned black and promptly peeled off).
According to the Westminster guys, eugenol is the culprit. Eugenol makes up 78% of clove oil and in an in vitro test on human fibroblasts was "highly cytoxic at concentrations as low as 0.03%". That doesn't sound good, but may be the Westminster boys are going out on a limb again. This time, however, they seem to be in good company and there's plenty of it.
There is evidence of dental assistants getting asthma from exposure to eugenol (it is commonly used by dentists to numb the gums), dental patients with mouth rashes, contact dermatitis from cosmetics and (even more likely) fragrance, and cytoxicity from eugenol as a food flavoring. On the other hand, it can be put to good use, according to one study that claims it inhibits the formation of melanoma.
Fortunately, it doesn't turn up in too many cosmetic products; so far I have only found it in Dr Hauschka, Dr Brandt and a bronzer by Guerlain.